The present invention relates to an electrophotographic lithographic printing plate, and more particularly to a lithographic printing plate which is improved in print staining.
A method for preparing a lithographic printing plate by the electrophotographic method is known. In general, an electrophotographic plate-making material bearing a photoconductive layer is uniformly charged, exposed imagewise and then wet or dry developed to form a toner image, which is then fixed. Thereafter, the material is treated with a desensitizing liquid or etching liquid to make non-image areas not bearing the toner image hydrophilic, whereupon a lithographic printing plate is obtained.
It is an essential requirement for a printing machine that ground contamination occurs less, but there is no printing machine fully satisfying this requirement. It is also required for a printing plate to be as free as possible from ground contamination and further to have a long press life. Conventional printing plates, however, fail to sufficiently satisfy the above requirements. For example, if the ratio of resin binder to zinc oxide particles in the photoconductive layer is decreased, the surface of the photoconductive layer is increased in desensitization properties, and ground contamination is reduced. On the other hand, the internal cohesive force of the photoconductive layer itself drops, that is, the mechanical strength becomes poor and the press life is shortened. On the contrary, if the ratio of resin binder to zinc oxide particle is increased, ground contamination is increased although the press life is increased.
Ground contamination is a phenomenon which appears on a whole printed paper and is correlated with desensitization properties of the surface of the photoconductive layer. While, spot contamination is one which appears spotted on a printed paper. The desensitization properties of the surface of the photoconductive layer are affected not only by the ratio of resin binder to zinc oxide in the photoconductive layer. Japanese Patent Publication No. 31011/75, Japanese Patent Application (OPI) Nos. 126406/74, 20735/79, and 68046/83, for example, describe that desensitization properties are effectively increased by changing or suitably choosing the type of the resin binder. The term "OPI" as used herein refers to a "published unexamined Japanese patent application".
Even if the resin binders described in the above-cited references are employed, ground contamination, spot contamination, plugging of dots, etc., and further, the press life are not sufficiently improved. In particular, ground contamination becomes marked in printing that utilizes color ink, which has been increasingly used in recent years. Moreover, if a number of plates are subjected to desensitization treatment by the use of an etching processor, the desensitization capability of the etching liquid is degraded, that is, contamination due to fatigue of the etching liquid sometimes occurs.
To reduce the ground contamination, a method of repeating treatment in an etching liquid which has been treated in an etching processor, that is, a so-called two-passage method, is known. It is empirically known that the two-passage method is effective in reducing ground contamination in a dot form. Thus when it is judged that ground contamination in a dot form would exert great influences on the value of printed matter, the two-passage method is often employed.
This two-passage method may be employed when the etching processor can be operated independently. However, in the case of an automatic print-making and printing system in which a plate-making processor and an etching processor, or an etching processor and a printer, or a plate-making processor, an etching processor and a printer are combined together into one assembly, the two-passage method using an etching processor is difficult to perform in practice. Thus with the use of automatic print-making and printing systems becoming more common, use of the two-passage method is often not possible from a practical viewpoint.